Has any of the food been floating on the surface while he was eating .... Fish sometimes take in air while taking in surface food ... Kinda like how colic is .... Sadly I know of people who use a syringe to suck the air out but you need to very precise with method or you run the risk of injuring the fish ....

a picture would help greatly, but are you sure its not that he has eaten too much nori at once, its known to cause lumps in the digestive tract and it may lead to bloating. Does the fish look distressed at all, as in fast gill movement, shaking of the head, spines out, dark brown colouration etc? If not id suggest NOT feeding a while (a few days, the other fish will manage a slight hunger strike in support) and seeing if it passes. But only if its caused by over eating of Nori.

I am no pro, i believe we all are on a journey of learning in this hobby. What i offer is from my research and my personal experience.
What works for me might not work for you and vice versa. It is always a great idea to do some research even after you get advice from others.
GOOD LUCK and Enjoy the Journey!!